The present invention relates to a new class of ductile irons, and to the process of forming such ductile irons.
Gray irons have carbon in the form of finely dispersed graphite flakes. These flakes allow the propagation of microscopic cracks when the alloy is placed under stress. Although gray irons are easily cast, they are weak in tensile strength.
In ductile irons, the carbon is in the form of small spheroids instead of flakes. These small spheroids act as "crack arresters" and stop the propagation of microscopic cracks when the iron is under stress. They allow ductile irons to have greater tensile strength than other irons, as well as other desirable properties. Several types of ductile irons may be produced, either "as-cast" or by means of special heat treatments.
Austempered ductile iron (ADI) is a known grade of material. ADI represents a special family of ductile iron alloys which possess almost twice the tensile strength of ordinary ductile irons, along with desirable characteristics of good elongation, toughness, good wear resistance and fatigue strength. These properties are achieved through special heat treatment called "austempering". For a general survey of ADI, see "Austempered Ductile Iron: Fact and Fiction", Kovacs, Modern Casting (March 1990), p.38-41.
The microstructure of austempered ductile iron is a matrix of acicular ferrite and high-carbon stable austenite. Austempered ductile iron castings are less brittle than common ductile iron, have improved strength-to-weight ratio, better surface detail and finish, improved machinability and reduced machining allowance.
The chemical composition of the base iron in ADI is similar to that of conventional ductile iron: about 3.6 C, 2.5 Si, 0.3 Mn, 0.015 maximum S and 0.06 maximum P. Alloying elements such as Cu, Ni, and Mo are added to the base composition. These elements are added not to increase strength or hardness, but to enhance heat treatability. The addition of the alloying elements does not affect the castability of the iron and does not increase the presence of casting defects. Large castings cool slower during quenching .and require more alloying than small castings.
All other casting process variables such as molding, nodularization, inoculation and pouring temperature are the same for ADI as they are for ductile iron. Alloying elements are often added to the ladle and the rest of the casting process is unaltered on a ductile iron line.
A typical ADI heat treatment cycle is shown in FIG. 1, where, according to Kovacs, the casting first is heat treated (A-B) to a temperature range of 1550.degree.-1750.degree. F. and held (B-C) at temperature for one to three hours. During this holding period, the casting becomes fully austenitic and the matrix becomes saturated with carbon.
After the casting is fully austenitized, it is quenched (C-D) in a quenching medium at a temperature range of 460-750 F. and held (D-E) at temperature for one-half to four hours. This temperature is called the "austempering" temperature. The austempering temperature and the holding time determine the final microstructure and properties of the ADI casting.
The effect of the austempering temperature on yield and tensile strength can be dramatic. High austempering temperatures result in high ductility, high fatigue and impact strengths and relatively low yield and tensile strengths. At low austempering temperatures, ADI displays high yield and tensile strengths, high wear resistance and lower ductility and impact strength. Strength increases rapidly by lowering the austempering temperature.
A high quenching rate during heat treatment is important so as to avoid formation of pearlite during quenching. The part must reach the targeted austempering temperature rapidly. Cooling profiles are also important, significantly affect material strength. After isothermal austempering, the casting is cooled to room temperature.
The five ASTM standard grades for ADI (ASTM 897-890) are as shown in Tables 1A and 1B. By comparison, a moderate grade wrought and tempered steel typically has tensile strength of about 192,000 psi, yield strength of about 162,000 psi, elongation of about 14.5% and hardness of about 385 BHN. However, parts made from wrought steel are heavier and more expensive to make and finish than are parts made from ADI.
TABLE 1A __________________________________________________________________________ THE FIVE ASTM STANDARD ADI GRADES (ASTM 897-90) IMPACT TYPICAL TENSILE YIELD ELONGATION ENERGY* HARDNESS GRADE STRENGTH STRENGTH (%) (FT-LBS) (BHN) __________________________________________________________________________ 1 125 80 10 75 269-321 2 150 100 7 60 302-363 3 175 125 4 45 341-444 4 200 155 1 25 388-477 5 230 185 N/A N/A 444-555 __________________________________________________________________________ *Minimum values **Un-notched Charpy bars tested at 72.degree. F. .+-. 7.degree. F.
TABLE 1B __________________________________________________________________________ THE FIVE ASTM STANDARD ADI GRADES (ASTM 897M-90) TENSILE YIELD IMPACT TYPICAL STRENGTH STRENGTH ELONGATION ENERGY* HARDNESS GRADE (MPa) (MPa) (%) (Joules) (BHN) __________________________________________________________________________ 1 850 550 10 100 269-321 2 1050 700 7 80 302-363 3 1200 850 4 60 341-444 4 1400 1100 1 35 388-477 5 1600 1300 N/A N/A 444-555 __________________________________________________________________________ *Minimum Values *Un-Notched Charpy Bars Tested At 22.degree. C. .+-. 4.degree. F.
Austempered ductile iron is used in many punishing applications. For example, it has been used in railroads for car wheels, suspension parts, track plates, latches, and other pans. ADI is also used in making heavy truck parts, including spring hangers, u-bolt plates, hubs, jack stand gears, mounting brackets, engine parts, and many other parts. Despite the useful properties offered by ADI, this material lacks the greater combined tensile strength and ductility of the more expensive wrought and tempered steels.
It is therefore an object of the present invention to provide a class of austempered ductile iron having a higher combination of tensile strength and ductility than previously known, and which may be substituted for moderate grade tempered wrought steels. It is also an object of the invention to provide a class of austempered ductile iron that consistently achieves desired properties, including tensile strength and ductility. A further object of the invention is to provide a process for forming such a material. Other objects will be apparent upon review of the following.